We all know that in wedge shaped film fringes are equally spaced.Being a wedge shaped film Why does the fringe width decrease as order increase in Newton`s ring ?
You can think of the fringes as contour lines and with a constant increase in slope (increase in thickness of wedge) the contour lines (fringes) are equally spaced.
Think of a hill with constant slope.
In the Newton’s rings arrangement the thickness of the wedge (distance between the bottom of the men’s and the top of the base plate) increases more rapidly as one moves further from the centre.
This more rapid increase in thickness results in the fringes (contour lines) becoming closer together as one moves further from the centre of the lens.
Think of a hill which becomes steeper as one climbs it.